1. Field of the Invention
The present invention relates to a direct liquid feed fuel cell stack and method, and more particularly, to a sealing structure between bipolar plates and an MEA (Membrane Electrode Assembly) included in a Direct Methanol Fuel Cell.
2. Description of the Related Art
A Direct Methanol Fuel Cell (DMFC) is a device for producing electricity. The DMFC has high energy density and power density by an electro-chemical reaction between an organic compound fuel such as methanol or ethanol and an oxidant, e.g., oxygen. Because it directly uses a liquid fuel, the DMFC does not require peripheral devices such as a fuel reformer, and has the advantages of easy storing and instant supplying of the liquid fuel.
As depicted in FIG. 1, a single cell of the DMFC includes a membrane electrode assembly (MEA) having an electrolyte membrane 1 between an anode 2 and a cathode 3. The anode 2 and the cathode 3 include fuel diffusion layers 22 and 32 for fuel supply and diffusion, catalyst layers 21 and 31 for oxidation and reduction reactions of the fuel, and electrode supporting layers 23 and 33, respectively. A catalyst formed of a precious metal having a superior catalytic characteristic at a low temperature, e.g., platinum, is used for an electrode reaction. Alternatively, in order to avoid catalyst poisoning by a by-product of the reaction, e.g., CO, an alloyed catalyst containing a transition metal such as ruthenium, rhodium, osmium, or nickel is used. A wet-proof carbon paper or a carbon cloth for easy fuel supply and discharge of the reaction products is used for an electrode supporting layer. An electrolyte membrane is a polymer membrane having a thickness in a range of 50˜200 μm. A proton exchange membrane with ionic conductivity may be used as the electrolyte membrane.
The electrochemical reaction in the DMFC, which uses a mixture of methanol and water as fuel, includes an anode reaction where the fuel is oxidized and a cathode reaction where oxygen is reduced.
Each reaction can be described as:CH3OH+H2O=CO2+6H++6e−  (Reaction 1—Anode reaction) 3/2O2+6H++6e−=3H2O   (Reaction 2—Cathode reaction)CH3OH+ 3/2O2=2H2O+CO2   (Reaction 3—Overall reaction)
At the anode 2 where an oxidation reaction (reaction 1) occurs, one carbon dioxide, six protons, and six electrons are produced. Produced protons move to the cathode 3 through a proton exchange membrane 1. At the cathode 3 where a reduction reaction (reaction 2) takes place, water is produced by the reduction reaction between protons, electrons transferred from an external circuit, and oxygen. Accordingly, as the result of an overall reaction (reaction 3), water and carbon dioxide are produced from the reaction between methanol and oxygen.
A theoretical voltage from a single cell of a DMFC is approximately 1.2 V. However, an open circuit voltage at ambient temperature and atmospheric pressure falls below 1.0 V, due to a voltage drop caused by an activation overvoltage and a resistance overvoltage. In reality, an actual operating voltage lies in the range of 0.4˜0.6 V. Therefore, to obtain higher voltages, a plurality of single cells, connected in series, are required.
A fuel cell stack is formed by stacking several single cells, connected in series. Adjacent single cells are electrically connected to each other by an electric conductive bipolar plate 4 interposed between the single cells.
The bipolar plate 4 can be formed of a graphite block which has high mechanical strength, high electrical conductivity, and good machining property. A block of composite material, containing a metal or a polymer, can also be used as the bipolar plate 4.
Fuel flow channels 41 and 42 are formed on both faces of the bipolar plate: channels 41 supply fuel, e.g., methanol on one face contacting the anode 2, and flow channels 42 supply air on the opposite face contacting the cathode 3. A bipolar plate 4 interposed between the fuel cell stack has a channel for supplying fuel on one face and a channel for supplying air on an opposite face. At the top and bottom of the fuel cell stack, end plates (not shown), e.g., monopolar plates, are placed. Thus, flow channels 41 and 42 supply fuel or air to an adjacent single cell formed on the end plate.
FIG. 2 illustrates a surface of a conventional bipolar plate having liquid fuel flow channels. FIG. 3 illustrates a gasket 5 attached to the surface of the bipolar plates, depicted in FIG. 2.
Referring to FIG. 2, a plurality of fuel channels 41 having a serpentine shape and openings at their upper surface are formed on an electrode region 47 of a conventional bipolar plate 4, where an MEA will be located. At an outer region of the electrode region 47, manifolds 46 connect to an inlet and an outlet of the fuel channels 41, and fuel flow holes 43a, 43b, 44a, and 44b connect to the manifolds 46 through the bipolar plate 4. The fuel flow holes 43a, 43b, 44a, and 44b include an inlet 43a and an outlet 43b of liquid fuel, and an inlet 44a and outlet 44b of an oxidant. The manifolds 46 connecting the liquid fuel flow holes 43a and 43b to the plurality of fuel channels 41 are formed inside the bipolar plate 4, without being exposed to a surface of the bipolar plate 4.
Referring to FIG. 3, the electrode region 47 and fuel flow holes 43a, 43b, 44a, and 44b, of the bipolar plate 4, are opened in the gasket 5.
FIG. 4 is a cross-sectional view of the gasket 5 and an MEA, located on the bipolar plate 4, taken along a line A-A in FIG. 2.
Referring to FIG. 4, the MEA is placed on the fuel channels 41 and 42, i.e., the electrode region, and the gasket 5 covers the rest of the bipolar plate 4, except for the fuel flow holes 43a, 43b, 44a, and 44b. The gasket 5 prevents the leaking in and out of the fuel and air.
The conventional bipolar plate 4, depicted in FIG. 4, has a relatively thick thickness of about 5˜10 mm since the manifold 46 is formed inside the bipolar plate 4. Reference numeral 46a refers to a portion of the bipolar plate 4 covering an upper face of the manifold 46.
For a smaller and lighter fuel cell, a thickness of the bipolar plate should be thinner, such as approximately 1˜2 mm. Accordingly, the bipolar plate 4, depicted in FIGS. 2 through 4, is not available for use in the smaller and lighter fuel cell. For this reason, a structure exposing the manifolds has been proposed. An example of this structure is described in U.S. Pat. Nos. 6,284,401, 5,879,826, and 6,146,780.
FIG. 5 schematically illustrates a bipolar plate depicted in U.S. Pat. No. 6,284,401. The same reference numerals are used to refer to like elements in FIGS. 1 through 4, and corresponding detailed descriptions are omitted.
Referring to FIG. 5, a plurality of fuel channels 41, having a serpentine shape and openings at their upper surface, are formed on an electrode region 47, of a bipolar plate 4, where an MEA will be placed. At another region of the electrode region 47, manifolds 46′ respectively connect to an inlet and an outlet of the fuel channels 41, and fuel flow holes 43a, 43b, 44a, and 44b connecting to manifolds 46′, through the bipolar plate 4, are formed. The fuel flow holes 43a, 43b, 44a, and 44b include an inlet 43a and an outlet 43b of liquid fuel, and an inlet 44a and outlet 44b of an oxidant.
The manifolds 46′, connecting the fuel flow holes 43a and 43b to the plurality of fuel channels 41, are formed on the bipolar plate 4, exposed on the surface of the bipolar plate 4.
FIG. 6 is a cross-section view of a gasket 5 and an MEA, which are located on the bipolar plate 4, taken along the line B-B in FIG. 5.
Referring to FIG. 6, the MEA is placed on the fuel channels 41 and 42, i.e., on the electrode region 47, illustrated in FIG. 5, with the gasket 5 being located on the rest of the electrode region 47 except for fuel flow holes 43a, 43b, 44a, and 44b. The gasket 5 is also placed on the manifold 46′ for preventing fuel leakage.
However, for fabricating a fuel cell stack, the plurality of bipolar plates 4 and a plurality of MEAs become compressed under high pressure. In this case, since the gasket 5, located on the manifold 46′, is now unsupported the gasket 5 is prone to bend, which causes leaking of the fuel through an upper part of the gasket 5. As illustrated in FIG. 4, previous methods supported gasket 5 with a portion of the bi-polar plate 4.
In order to solve the bending problem of the gasket 5, a bridge piece can be installed, connecting to the gasket 5 on the manifold 46′, reducing a thickness of the bipolar plate 4, as discussed in U.S. Pat. No. 6,410,179.
However, due to the difficulties in manufacturing and installation, and the weak mechanical strength of the bridge piece, fabricating of a fuel cell stack by pressing the gasket and the bipolar plates at a high pressure is not easy or simple. Moreover, this high pressure method is difficult to apply for fabricating a thin bipolar plate having a thickness of 1˜2 mm, which is an aim of the present invention.